1. Field of the Invention
This invention relates to liquid crystalline compounds and mixtures.
2. Background Description
Liquid crystals have gained considerable importance as dielectrics in indicating devices, since the optical properties of such substances can be influenced by an applied voltage. Electro-optical indicating devices which utilize liquid crystal cells are well known to the person skilled in the art.
Liquid crystals must satisfy a number of requirements in order to be suitable as dielectrics for electro-optical indicating devices. For example, the liquid crystals must have a high chemical stability towards environmental factors (e.g. heat, air, moisture and the like), must be photochemically stable and colourless, must have short response times, must not be too high a viscosity, must have a nematic or cholesteric-type mesophase in all temperature ranges in which the liquid crystal cell is to be operated, and must give a good contrast. Other properties such as, for example, the threshold potential, the dielectric anisotropy and the electrical conductivity must fulfill different conditions depending on the type of cell which is used.
Nematic and cholesteric liquid crystals with negative anisotropy of the dielectric constants (.DELTA..epsilon.=.epsilon..sub..parallel. -.epsilon..sub..perp. &lt;O, .epsilon..sub..parallel. signifying the dielectric constant along the longitudinal molecular axis and .epsilon..sub..perp. signifying the dielectric constant perpendicular thereto) can be orientated in an electric field with their longitudinal molecular axes perpendicular to the field direction. The effect of this orientation is known and is used to control optical transmissivity in various liquid crystal indicators, for example in liquid crystal cells of the light scattering type (dynamic scattering), of the so-called DAP type (deformation of aligned phases) or of the guest/host type (guest host interaction).
The "guest/host cell" comprises essentially a condenser, at least one electrode being transparent and the dielectric being formed from a nematic or cholesteric liquid crystal which contains one or more dichroic colouring substances. Since the colouring substances used mainly have positive dichroism, i.e. the transition moment of the absorption of visible light lies approximately in the direction of the longitudinal molecular axis, the orientation of the liquid crystal with the molecular axes parallel to the surface of the plates generally corresponds to the coloured state and the homeotropic orientation (longitudinal molecular axes perpendicular to the surface of the plates) generally corresponds to the colourless state of the cell. When a liquid crystal with positive dielectric anisotropy is used, its homogeneous orientation (which is achieved by treating the surface of the plates) is arranged homeotropic by the application of a voltage, i.e. the cell is swiched from "coloured" to "colourless." In this manner colourless symbols are shown on a coloured background. On the other hand, when a liquid crystal with negative dielectric anisotropy is used, its homeotropic orientation (by treating the surface of the plates) is arranged parallel to the electrode surfaces by the application of a voltage, whereby the reading of coloured image elements on a colourless background is made possible.
The customary, static operation of liquid crystal indicating devices has in the past been replaced to an increasing extent by the so-called multiplex control. In this case there is mainly used an amplitude-selective multiplex procedure, whereby, however, by the procedures usually used, in general only multiplex ratios of about 1.8 to 1.10 have been attained. However, for the improvement of the multiplex ratio in the multiplex control of liquid indicators, especially of rotation cells and guest/host cells, a two-frequency matrix addressing procedure has been proposed (e.g. German Offenlegungsschriften Nos. 2 856 134 (Great Britain Pat. No. 2 013 014) and 2 907 940 (Great Britain Pat. No. 2 020 075). This makes use of the fact that the dielectric anisotropy of liquid crystals, which upon application of a low frequency voltage have a positive anisotropy of the dielectric constants, is negative in the case of high frequencies. In order to maintain the capacitative loss low, the "cross-over frequency" f.sub.c (dielectric relaxation frequency at which .epsilon..sub..parallel. =.epsilon..sub..perp.) of such liquid crystals should be as low as possible and should not lie above about 20 kHz. Further, the absolute value of the dielectric anisotropie should be as large as possible not only below but also above the cross-over frequency. It has, however, been found that the substances, which are especially suitable for the two-frequency procedure, at frequencies above the cross-over frequency generally have a smaller absolute dielectric anisotropy than below the cross-over frequency. This disadvantage can be eliminated by adding compounds with negative dielectric anisotropy and suitable relaxation behaviour.
A series of liquid crystalline compounds with weakly negative dielectric anisotropy has already been synthesized. On the other hand, still relatively few liquid crystal components with large negative anisotropy of the dielectric constants are known. Moreover, the latter generally have disadvantages, such as, for example, poor solubility in mixtures, high viscosity, high melting points, strong smectic tendencies and chemical instability. There accordingly exists a need for more compounds with negative dielectric anisotropy which further improve the properties of liquid crystal mixtures so that they may be used for an even wider variety of electro-optical purposes.